The present invention relates to an apparatus for detecting optically defects in an object by projecting a light beam onto the object and by detecting a variation in the light beam reflected by the object.
In a semiconductor manufacturing field, a quality of a semiconductor wafer must be checked. If there are defects in a ground surface of the semiconductor wafer, a property of finally obtained semiconductor devices might be deteriorated or damaged. Also in the field of manufacturing video and audio discs, an original disc made of glass should not have defects. If the original glass disc has defects in its ground surface, a number of video and audio discs manufactured by the original disc might become defective. Therefore, it is necessary to check the surface condition of the semiconductor wafer and the glass disc after their surfaces have been ground into mirror surfaces.
Usually the checking operation is carried out visually by experienced persons. The visual checking can discriminate the defects from others precisely, but it is difficult to obtain a high reliability due to difference between respective persons and mistakes inherent to the human beings. Further, the operations are subjected to fatigue and efficiency of checking is limited. Moreover, it is difficult to sort or select the detected defects in accordance with their properties or characteristics. Therefore, there has been required to develop an apparatus for detecting defects with high efficiency and reliability.
Heretofore, there have been developed various methods for detecting small defects and fine particles. In the simplest method, the human eyes are replaced by a television camera and an output video signal from the television camera is suitably processed by an electrical circuit to derive a defect signal. However, presently available television cameras do not have a sufficiently high resolving power for detecting small defects. Several types of apparatuses for detecting fine dusts or particles in air have been commercially available. However, these apparatuses use an incandescent lamp as a light source and thus, the maximum resolving power is limited only to about 0.5 .mu.m and even the highest grade apparatus has the resolving power of 0.2 .mu.m to 0.3 .mu.m. In an apparatus for detecting smaller particles, use is made of a laser light source. Since the laser light source can generate a coherent parallel light beam having a very small cross section, a difference between scattered or diffracted light and non-scattered light becomes large and useful information can be derived from a phase component.
FIG. 1 is a schematic view showing the known apparatus for checking a surface condition of a semiconductor wafer by means of the laser light. In FIG. 1, a laser light beam 1 emitted from a laser light source L is reflected by a mirror 2 and is focussed by means of an objective lens 3 onto an object 4 to be checked, i.e. a semiconductor wafer. In order to collect a light flux 7 scattered by a defect in the object 4, a condenser lens 5 is arranged beside the objective lens 3. The collected light flux is made incident upon a light detector 6 which produces an electric signal representing a defect. In order to move the incident light beam with respect to the object 4, the mirror 2 may be swung or the object 4 may be rotated or moved linearly. In this manner the whole surface of the object 4 can be scanned.
However, in such a known apparatus the condenser lens 5 can collect only a small part of the light flux scattered in all directions and therefore, it is difficult to obtain the signal having a high S/N and a detection accuracy is low. Moreover, in order to collect a larger amount of the scattered light, the objective lens 3 must be spaced from the object 4 by a larger distance and must have a longer working distance, and in order to project the bright light beam onto the object 4, the objective lens 3 must have a larger diameter or aperture. Under the above circumstances, in the known apparatus the freedom in designing the objective lens 3 is limited to a great extent.